Alcohols have been prepared from olefins by the oxo reaction using cobalt carbonyl catalyst either unmodified or modified with a coordinating ligand such as a phosphine. The alcohol product depends on nature of the starting olefin, the reaction conditions, and nature of the cobalt catalyst system. The major side products of the cobalt system are paraffin (from hydrogenation of starting olefin) and heavies. The phosphine modified cobalt system produces mostly paraffin as a side product whereas the unmodified system gives mostly heavies. In either case the alcohol yield is around 80% of the theoretical value.
More recently, processes have been developed which use rhodium as the hydroformylation catalyst. Most of these use phosphorus containing ligands as catalyst modifiers; however, some do not. Most of the published information about the rhodium catalyst system deals with converting alpha olefins to aldehydes and subsequently to alcohols that contain very high proportions of the linear isomer.
One of these rhodium catalyst systems is concerned with a process for making n-butylaldehyde from propene with a rhodium triphenyl phosphine catalyst system. That process uses a large excess of the phosphine ligand relative to the amount of rhodium present. In general, most rhodium catalyzed oxo systems use a rhodium concentration, based on olefins, in the range of 100-1,000 parts per million based on the feed.
Since rhodium is extremely expensive the rhodium catalyst system suffers a significant economic disadvantage.
I have discovered that recycled rhodium in small concentrations can be used as a catalyst in hydroformylation processes which employ internal olefins to get a substantial amount of linear and 2-methyl-branched aldehydes and/or alcohols. Still further, I have found that the rhodium can be recovered for recycling using a relatively simple flash distillation, and, even more unexpectedly, that the recycled rhodium is a more active catalyst than the original.
Rhodium has been known in the art to be useful for the preparation of aldehydes and alcohols from olefins using hydroformylation. Two U.S. patents which depict this are U.S. Pat. No. 4,299,990 and U.S. Pat. No. 3,933,919. Both of these patents, however, use rhodium in large concentrations. Specifically, in U.S. Pat. No. 4,299,990 the rhodium is in concentrations of 0.01% per weight of olefin, or better described in terms of parts per million, as 100 parts per million based on the olefin feed. U.S. Pat. No. 3,933,919 calls for rhodium in concentrations of 1,000 parts per million. Not only does the instant invention necessitate much smaller concentrations of rhodium but it also would not operate at concentrations this large. Available relevant data indicates that at higher rhodium concentrations the product yield from hydroformylation of internal olefins will be mainly heavily-branched aldehyde and alcohol material. Thus, these two patents teach away from the instant invention since the instant invention uses small concentrations of rhodium to produce aldehydes and alcohols that overall are more linear. More specifically, there is a substantial portion of the product that is linear and 2-methyl-branched aldehyde and alcohol.
U.S. Pat. No. 4,299,990 calls for a temperature range from 80.degree. C. to 160.degree. C. and U.S. Pat. No. 3,933,919 provides for a temperature range of 50.degree. C. to 150.degree. C.; and U.S. Pat. No. 4,148,830 provides for a temperature range of 50.degree. C. to 145.degree. C. These temperature ranges are, overall, too low for a feed stock of internal olefins, as in the case of the instant invention, when a more linear product is desired. When internal olefins are used, higher temperatures are necessitated.
In contrast, using temperatures called for in the instant invention, internal olefins will undergo extensive isomerization, and as a result, a larger variety of isomeric aldehyde are obtained. At the higher temperatures, the process becomes even more selective to the production of more linear isomers. Even more surprising and desirable is the higher concentration of linear aldehyde in the product. Much more linear aldehyde is formed than would be expected from an equilibrium mixture of olefin isomers as long as these high temperatures are maintained. Pressure must be carefully controlled. In the instant invention, higher pressures are necessitated to prevent catalyst decomposition at the temperatures required. Thus, due to carefully controlled inter-relating conditions, the instant invention enables hydroformylation of internal olefinic product with low rhodium concentrations which will yield a more linear product. The advantages of the instant invention are illustrated by contrast to previous art. The present invention provides the economic advantage of having low rhodium concentrations, the advantage of being able to use an exclusively internal olefin feed, and also the advantage of obtaining a more linear product. Other advantages, however, will become apparent to those skilled in the art as the description proceeds.